Single (I.P. or S.C.) injections of lead salts, (acetate or nitrate), at dosage levels producing a LD50 response, will be given to pregnant mice at daily intervals from the 8th to 16th days of gestation. Chick embryos will be treated with predetermined sublethal doses at daily intervals from the 2nd to 8th days of incubation. The pathological effects of such treatment on the morphological development of vertebrate embryos will be investigated. Other avian and mammalian embryos will be exposed to previously determined teratogenic doses of lead and will receive concurrently doses of thymidine H3, uridine H3 or leucine H3. This method will allow us to determine whether or not lead effects biochemical activities such as DNA, RNA and protein synthesis in the developing central nervous system. The quantitative and qualitative distribution of lead in the developing fetus will be studied by atomic absorption spectrophotometry and radioautography.